1. Field of the Invention
The invention relates to a method for rapid cleaning of tungsten which coats Chemical Vapor Deposition chambers. In particular, the method uses gas phase chemistry and a single gas or a combination of hydrogen bromide, bromomethane, dibromomethane or bromoform to clean tungsten off the reactor surface. The method uses the chamber""s existing ports and vacuum system, is fast and easy and eliminates the need for an expensive plasma excitation system and plasma cleaning.
2. Description of the Prior Art
Chemical vapor deposition (hereinafter CVD) methods are used broadly in the manufacture of semiconductor wafers. CVD involves exposing a semiconductor wafer to a reactive gas under carefully controlled conditions of elevated temperatures, sub-ambient pressures, and uniform reactant gas flow rate so that a thin, uniform layer of film is deposited on the surface of the substrate. Several types of CVD processes are known, such as, for example, atmospheric CVD, low-pressure CVD, reduced-pressure CVD and plasma enhanced CVD.
A commonly used CVD system is a single-wafer CVD system utilizing a high-throughput CVD chamber. This is a result of the more stringent standards of film quality and increasing wafer sizes utilized in recent years.
For processing, typically, a CVD chamber is first heated in a range of 300 to 1,000 degree Celsius. A semiconductor substrate is mounted on a holding piece, called a susceptor, which is typically made of anodized aluminum. Then, processing gases are flowed into the chamber while the chamber pressure is regulated to an optimum level for achieving a uniform layer of film. The gases react to form a deposition layer on the surface of the wafer.
In a conventional deposition process, reactant gases enter the reaction chamber and produce films of various materials on the surface of a substrate for various purposes such as for dielectric layers, for insulation layers, etc. The various electronic materials deposited include refractory metals such as titanium, tungsten and their suicides. These refractory metals and their silicides have found widespread use in the manufacture of very large-scale integrated circuits. Tungsten (W), in particular, because of its chemical stability and its ease of film formation by CVD, is used extensively for the formation of electrical contacts to silicon device elements. Tungsten silicide (WSi2) is frequently selectively formed over exposed silicon surfaces by depositing a tungsten layer, annealing to react the layer with the silicon areas and etching away the unreacted tungsten, leaving tungsten silicide only on the silicon.
In these film deposition processes, most of the material from the reactant gases is deposited on the substrate surface. However, it is inevitable that material is also deposited on other surfaces inside the chamber. During the deposition process, flakes of tungsten from these surfaces can fall onto the wafer and cause a high density of defects. In order to maintain a consistent process and reduce defects, the walls of the chamber are cleaned between several lots or as part of a periodic maintenance schedule. Often, the cleaning is done after every twenty-five wafers in order to maintain high efficiency and reduce defects. This cleaning process typically uses plasma cleaning, often-containing hexafluoroethane, argon and oxygen. This method requires an expensive plasma excitation system where electrodes are connected to a RF power generator and the gas mixture is excited to plasma. The plasma is then used to bombard the chamber walls to remove the tungsten coating. Some variability in the cleaning capability of this system exists as its efficiency is relative to the pressure applied in the chamber. Methods using plasma cleaning are described in U.S. Pat. No. 5,902,403 issued to Aitani and U.S. Pat. No. 6,047,713 to Robles.
Alternatively, a wet clean method can be employed where the chamber is opened, taken apart, cleaned by wiping the chamber walls down and then baked at high temperature. The reassembled chamber is then pumped down to a high vacuum to remove any cleaning solvent. This is a very laborious and time consuming process which leads to poor throughput and increased cost of manufacturing. Furthermore, after each manual cleaning process, the entire exhaust flow control system must be recalibrated in order to resume production.
Methods using non-plasma gases such as that disclosed by Xi in U.S. Pat. No. 5,926,743 make use of an etchant gas (chlorine tri-fluoride) diluted with an inert gas or carrier to remove particles and buildup from chamber walls. Etchant gases require careful temperature and pressure controls to prevent damage to the substrate or the chamber.
It has now been found that a fast and effective in-situ method for cleaning tungsten deposited on the chamber walls of CVD chambers after use can be provided. In particular, the method eliminates formation of plasma with any excitation frequency, including microwave, from the cleaning process and, instead, uses a combination of appropriate gases that reduces the cost associated with the process.
The present invention provides an improved method for cleaning tungsten deposited onto the walls of CVD deposition chambers, without necessitating modifications to the deposition chamber. Additionally, the ratio of gases can be adjusted continuously in order to maximize formation and removal of tungsten bromide species. Further the process does not require disassembly of the chamber prior to cleaning so loss of production time is minimized.
Cleaning of the CVD chamber is generally done after the silicon wafers have been removed to avoid removing desired depositions from the wafer itself. Prior to cleaning, an inert gas such as nitrogen or argon can be used to purge the chamber of any interfering species that may reduce the efficient removal of tungsten from the chamber walls. After clearance of any purge gas, hydrogen bromide or dibromomethane is then pumped into the chamber alone or in combination with one or more of the bromomethanes including bromoform, dibromomethane or bromomethane. Unlike plasma cleaning, carrier gases are not required in the use of these compounds.
It has been found that either hydrogen bromide or dibromomethane alone can remove the tungsten from the chamber walls. In a preferred embodiment, a combination of bromoform is added to the HBr and the combination reacts with the tungsten on the chamber walls to form various tungsten-bromium species, including WBr2, WBr4 and WBr5 that are then pumped away using the chamber""s vacuum system. The HBr in combination with the other gases has been found to substantially increase the efficiency of the cleaning operation and to reduce the amount of time required for the cleaning step. In another embodiment, a combination of the three methano-bromium species is used with hydrogen bromide to modify removal of tungsten. In a third embodiment, bromoform or a combination of two or all three of the bromomethanes is pumped into the CVD chamber over a range of ratios to clean various tungsten-bromium species. After cleaning, an inert gas such as nitrogen is used to purge the chamber completely of reactive bromine in order to prevent interference of deposition during production of substrate.